This invention relates to the art of applying an organic coating to a metal surface. Specifically, it relates to the application of a water-based organic coating composition to provide corrosion resistance and paint receptivity to a metal surface. Even more specifically, this invention results to a composition and process for imparting an organic coating on a backplate or tinplate can surface.
Present and future markets for metal cans depend primarily upon aluminum, tin-plated steel, and blackplate materials. Methods of can manufacture include three-piece methods wherein a separate top and bottom are combined with a seamed sidewall structure to form the completed can and two-piece methods wherein a unitary sidewall and bottom structure is formed by cold-forming from a metal blank and the top attached thereto to form the completed can. While the particular coating requirements vary depending upon the manufacturing method and/or material, in general, it is necessary to apply a relatively non-porous organic coating to the metal surface in order to:
(1) impart sufficient corrosion resistance to the metal surface; PA1 (2) provide a suitable base for decorative paints and over-varnishes; and PA1 (3) provide a suitable barrier between the metal surface and the food, beverage or other contents for which the can is designed. PA1 (1) form unitary sidewall and bottom, PA1 (2) clean with a hexavalent alkaline chromium cleaner, PA1 (3) rinse, PA1 (4) optionally apply wash coat, PA1 (5) dry, PA1 (6) apply outside decorative lacquer, PA1 (7) bake, PA1 (8) apply interior sanitary lacquer, PA1 (9) bake. PA1 (a) an epoxy resin having an average molecular weight of about 900 to 2900 and about 2.7 to 11.6 hydroxy equivalents per mole; PA1 (b) an alkylated melamine-formaldehyde resin in a weight ratio of epoxy resin to melamine-formaldehyde resin of from 2:1 to 20:1; PA1 (c) an acid catalyst in an amount sufficient to effect cross-linking of the epoxy resin and malamine-formaldehyde resin; and PA1 (d) a block copolymer of the general formula ##STR1## having a Hydrophilic-Lipophilic Balance (HLB) in excess of 22 and a molecular weight in the range of about 5000 to 14,000 in an amount of about 2-12 weight % of the total of the epoxy and melamine-formaldehyde resin is suitable for forming an aqueous emulsion and for treating a ferrous or tin surface to form a protective coating thereon when employed at 5 to 85 weight % of components (a) through (d).
Environmental considerations have had a substantial impact upon the development of new techniques and compositions for the manufacture of cans. Specifically, conventional processes which employ chromium chemicals and which employ large quantities of volatile organic solvents are environmentally objectionable. Accordingly, it is now desirable to develop compositions and techniques which result in cans of acceptable quality without incurring the environmental penalties inherent in employing chromium and/or volatile organic solvents.
The application of coatings to two-piece cans is particularly critical because of the fact that the coatings must be applied to the unitary bottom and sidewall structure which makes access more difficult. In the two-piece can manufacturing process for tinplate cans, the following steps would be employed for a beverage can:
In the case of blackplate cans, the procedure would be similar with an optional application of a conversion coating and a rinse following the stage (3) rinse. Further, it is often necessary to apply two layers of sanitary lacquer to avoid undue porosity of the surface coating.
The conventional technique for applying the interior sanitary lacquer to the can is via an airless spray device. This requires that the cans be individually sprayed to obtain a uniform coating. On the other hand, during the cleaning and rinsing stages the cans are not separated for individual treatment. It would therefore be desirable to accomplish the application of the exterior base coat and an interior sanitary lacquered coat via mass treatment of cans with a wash coat in a can washer type of apparatus rather than treating each of the cans individually.
A number of water-borne coatings have been developed in recent years. See, for example, U.S. Pat. Nos. 3,908,049; 3,922,451; 3,968,311; and 3,996,182.